1. Field of the Invention
The present invention relates to a composition for a resist underlayer film useful as a composition for an anti-reflection film used in microfabrication in manufacturing of a semiconductor device and the like, and to a resist patterning process using this, suitable for exposure by a KrF excimer beam (248 nm), an ArF excimer beam (193 nm), an F2 laser beam (157 nm), a Kr2 laser beam (146 nm), an Ar2 laser beam (126 nm), a soft X-ray (EUV, 13.5 nm), an electron beam (EB), and an X-ray.
2. Description of the Related Art
As LSI is progressing toward a higher integration and a faster speed in recent years, further miniaturization of a pattern rule is required. Under such a movement, a lithography using a light exposure, which is a widely used technology today, is reaching a limit of its resolution power inherent to a wavelength of a light source.
As a light source for a lithography used in a resist patterning process, light-exposures with a g-line (436 nm) and an i-line (365 nm) have been widely used. For further miniaturization, a method in which an exposing light is shifting toward a shorter wavelength has been considered to be effective. Accordingly, in place of an i-line (365 nm), a KrF excimer laser (248 nm), which emits a shorter wavelength than the i-line, has been used in a mass production process for a 64 Mbit DRAM. However, for production of DRAM with an integration of 1 G or more, which requires a further miniaturized process technology (processing dimension of 0.13 μm or less), a light source with a further shorter wavelength is required, and thus, a lithography using an ArF excimer laser (193 nm) has been investigated particularly.
On the other hand, it has been known in the past that a bilayer process is excellent in formation of a pattern having a high aspect ratio on a nonplanar substrate. To develop a bilayer resist film by generally used alkaline developer, a silicone polymer having a hydrophilic group such as a hydroxy group and a carboxyl group is necessary.
As a silicone-type positive resist composition of chemically amplified type, a silicon-type positive resist composition of chemically amplified type using a base resin, which is obtained by protecting a part of a phenolic hydroxy group of polyhydroxybenzyl silsesquioxane, a stable alkaline-soluble silicone polymer, with a t-Boc group, together with an acid generator is proposed for a KrF excimer laser (see, for examples, Japanese Patent Laid-Open (kokai) No. H6-118651 and SPIE Vol. 1925 (1993), p. 377). For an ArF eximer laser, a positive resist composition based on a silsesquioxane whose cyclohexyl carboxylic acid is substituted with an acid-labile group is proposed (see, for example Japanese Patent Laid-Open (kokai) No. H10-324748 and Japanese Patent Laid-Open (kokai) No. H11-302382, and SPIE Vol. 3333 (1998), p. 62). Further, a positive resist composition based on a silsesquioxane having a hexafluoroisopropanol as a soluble group is proposed for an F2 excimer laser (see, for example, Japanese Patent Laid-Open (kokai) No. 2002-55456). The above-mentioned polymers contain a polysilsesquioxane having a ladder skeleton in their main chain made by polycondensation of a trialkoxy silane or a trihalogenated silane.
As a resist base polymer having a silicon pendant on its side chain, a polymer of a silicon-containing (meth)acrylate ester is proposed (see, for example Japanese Patent Laid-Open (kokai) No. H9-110938 and J. Photopolymer Sci. and Technol., Vol. 9, No. 3 (1996), p. 435-446).
A resist underlayer film in a bilayer process is formed of a hydrocarbon compound, which can be etched by an oxygen gas, and in addition, needs to have a high etching resistance because it becomes a mask when a substrate under it is etched. For etching by an oxygen gas, the film must be composed of only a hydrocarbon, not containing a silicon atom. In addition, the film needs to have a function as an anti-reflection film in order to improve a controllability of a line width of a silicon-containing resist film above it and to form less bumps on a pattern sidewall and to reduce collapsing of a pattern by a standing wave. Specifically, a reflectance from an underlayer film to a resist upperlayer film needs to be made 1% or less.
Here, calculation results of the reflectance till a film thickness of maximum 500 nm are shown in FIGS. 2 and 3. In FIG. 2, assumptions are made 193 nm for a wavelength of an exposure light, and 1.74 for an n-value and 0.02 for a k-value of the resist upperlayer film. A substrate reflectance is shown for the case where the k-value of the resist underlayer film is fixed at 0.3 with varying the n-value from 1.0 to 2.0 in a vertical axis and a film thickness from 0 to 500 nm in a horizontal axis. When a resist underlayer film with a film thickness of 300 nm or more is assumed in a bilayer process, there is an optimum value to make a reflectance 1% or less within the n-value range of 1.6 to 1.9, which is the same as or a little higher refractive index as compared with the resist upperlayer film.
In FIG. 3, a reflectance is shown in the case where the n-value of the resist underlayer film is fixed at 1.5 with varying the k-value from 0 to 0.8. When a resist underlayer film with a film thickness of 300 nm or more is assumed in a bilayer process, it is possible to make the reflectance 1% or less within the k-value range of 0.24 to 0.15. On the other hand, the optimum k-value of an anti-reflection film in a monolayer resist process used in a thin film with a thickness of about 40 nm is 0.4 to 0.5, which is different from the optimum k-value of a resist underlayer film in a bilayer process with a film thickness of 300 nm or more. This suggests that a further lower k-value, namely a further higher transparent resist underlayer film, is required in a resist underlayer film in a bilayer process.
As disclosed in SPIE Vol. 4345 (2001), p. 50, a copolymer of polyhydroxy styrene and an acrylate ester is investigated as a composition for a resist underlayer film for a 193 nm wavelength. Polyhydroxy styrene has a very strong absorption at 193 nm, and the k-value of itself is about 0.6, which is too high. Accordingly, the k-value is controlled in the vicinity of 0.25 by copolymerizing it with an acrylate ester whose k-value is almost zero.
However, an etching resistance of an acrylate ester in a substrate etching is low as compared with polyhydroxy styrene. In addition, in order to lower the k-value, a considerably large ratio of an acrylate ester needs to be copolymerized, thereby leading to considerable decrease in the resistance in the substrate etching. The etching resistance not only affects an etching rate but also a surface roughness after the etching. An increase in the surface roughness after the etching is an acute problem caused by copolymerization of an acrylate ester.
On the other hand, a trilayer process, which involves lamination of a resist upperlayer film of a monolayer resist not containing a silicon, under which a resist intermediate layer film containing a silicon, under which a resist underlayer film of an organic film, is proposed (see for example J. Vac. Sci. Technol., 16 (6), November/December 1979). Generally, a monolayer resist has a higher resolution as compared with a silicon-containing resist, and thus a monolayer resist with a high resolution can be used as an exposure imaging layer in a trilayer process. A spin-on glass (SOG) film is used as a resist intermediate layer film, and many SOG films are proposed.
An optimum optical constant of an underlayer film to suppress a substrate reflection in a trilayer process is different from that in a bilayer process. In a bilayer process a resist underlayer film is solely responsible for an anti-reflection effect, while in a trilayer process a resist intermediate layer film and/or a resist underlayer film may be responsible for the said effect, though there is no difference between the bilayer process and the trilayer process in the purpose to suppress the substrate reflection as low as possible, specifically 1% or less.
A composition for a silicon-containing layer having an anti-reflection effect is proposed in U.S. Pat. No. 6,506,497 and U.S. Pat. No. 6,420,088. Generally, an anti-reflection effect is higher in a multilayer anti-reflection film than in a monolayer anti-reflection film, and thus the former is used widely as an anti-reflection film in an optical composition. A high anti-reflection effect may be obtained by rendering an anti-reflection effect to both a resist intermediate layer film and a resist underlayer film.
If a silicon-containing resist intermediate layer film is rendered with an anti-reflection function in a trilayer process, an utmost anti-reflection effect as requested for a resist underlayer film in a bilayer process is not particularly necessary. In a trilayer process, the resist underlayer film is requested to have a high etching resistance in a substrate processing rather than to have the anti-reflection effect.
Accordingly, a novolak resin, which contains many aromatic groups and has a high etching resistance, has been used as a resist underlayer film in a trilayer process.
In FIG. 4, a substrate reflectance with a varied k-value of a resist intermediate layer film is shown.
A sufficient anti-reflection effect with the reflectance of 1% or lower may be obtained by setting the k-value of a resist intermediate layer film at low, i.e., 0.2 or less, and a film thickness appropriately.
Usually, in order to suppress the reflectance to 1% or less with a film thickness of 100 nm or less, the k-value of an anti-reflection film needs to be 0.2 or more (see FIG. 3). However, in a trilayer resist film in which a certain degree of reflection can be suppressed in the resist underlayer film, an optimum k-value for the resist intermediate layer film is less than 0.2.
In FIG. 5 and FIG. 6, an effect on the reflectance is shown when the thickness of a resist intermediate layer film and of a resist underlayer film are varied with the fixed k-value of the resist underlayer film at 0.2 and 0.6. The resist underlayer film with the k-value of 0.2 in FIG. 5 is assumed for the optimum case of the resist underlayer film in a bilayer process. The k-value of 0.6 for the resist underlayer film in FIG. 6 is near to the k-value of a novolak or polyhydroxy styrene at 193 nm wavelength.
A film thickness of a resist underlayer film changes with topography of a substrate, while a film thickness of a resist intermediate layer hardly changes, and thus it is assumed that an intended thickness can be obtained with application of a solution.
A higher k-value of a resist underlayer film (the case of 0.6) can suppress the reflectance to 1% or less with a thinner film. When the k-value of a resist underlayer film is 0.2 with a film thickness of 250 nm, a thickness of a resist intermediate layer film needs to be increased in order to make a reflectance of 1%. However, when a thickness of a resist intermediate layer film is increased like this, a load of the uppermost resist film is high during processing of the resist intermediate layer by dry-etching, which is not desirable.
FIG. 5 and FIG. 6 show reflection in a dry exposure with an NA of an exposure lens being 0.85. It suggests that a reflectance may be made 1% or less independent of a k-value of a resist underlayer film by optimizing an n-value, a k-value, and a thickness of a resist intermediate layer film in a trilayer process. On the other hand, by an immersion lithography, an NA of a projector lens exceeds 1.0, and incidence light angles not only to a resist but also to an anti-reflection film underneath the resist become shallow. An anti-reflection film suppresses a reflection by not only an absorption by itself but also a compensation action of a light interference effect. A light interference effect of a skew light is small so that reflection increases. A resist intermediate layer film is responsible for an anti-reflection by a light interference effect among the films in a trilayer process. A resist underlayer film is too thick to effect an anti-reflection by light interference compensation. A reflection from a resist underlayer film surface needs to be suppressed, and for that, it is required that the k-value of a resist underlayer film be less than 0.6 and the n-value be near to that of the resist intermediate layer film on it. When the k-value is too small thereby too high in transparency, reflection from a substrate becomes eminent so that an optimum k-value becomes between about 0.25 to about 0.48 for the case of an immersion exposure with NA of 1.3. The target of the n-value is near 1.7, which is the n-value of a resist in both an intermediate layer as well as a underlayer.
A benzene ring has a very strong absorption so that the k-value of a cresol novolak or polyhydroxy styrene is over 0.6. One of those having a higher transparency at 193 nm than a benzene ring and a high etching resistance is a naphthalene ring. For example, a resist underlayer film having a naphthalene ring or an anthracene ring is proposed in Japanese Patent Laid-Open (kokai) No. 2002-14474. According to our measurement, the k-values of a naphthol copolycondensed novolak resin and a polyvinyl naphthalene resin are in a range of 0.3 to 0.4.
The n-values of a naphthol copolycondensed novolak resin and a polyvinyl naphthalene resin at 193 nm is low, i.e., 1.4 for a naphthol copolycondensed novolak resin and further lower, i.e., 1.2, for a polyvinyl naphthalene resin. For example, an acenaphthylene polymer shown in Japanese Patent Laid-Open (kokai) No. 2001-40293 and Japanese Patent Laid-Open (kokai) No. 2002-214777 has the n-value of 1.5, and the k-value of 0.4, which is close to targeted values. A transparent underlayer film with a high n-value and a low k-value and having a high etching resistance is desired.
In Japanese Patent Laid-Open (kokai) No. 2007-199653, a composition for a resist underlayer film having a bisnaphthol group, which has the n-value and the k-value near to their target values and with an excellent etching resistance, is proposed.
In the case when an underlying substrate to be processed is nonplanar, the nonplanarity needs to be made flat by a resist underlayer film. Flattening of the resist underlayer film will suppress a change in thickness of a resist intermediate layer film formed on it and of a photoresist film, which is a resist upperlayer film, thereby enlarging a lithography focus margin.
However, in an amorphous carbon underlayer film formed by a chemical vapor deposition (CVD) method using a raw composition gas such as methane, ethane, and acetylene, it is difficult to fill in the nonplanarity to flat. On the other hand, formation of a resist underlayer film by a spin coat method has an advantage of filling in substrate's concavity and convexity. Further, in order to improve gap filling characteristics of a coating composition by application, as shown in Japanese Patent Laid-Open (kokai) No. 2002-47430, a method in which a novolak having a low molecular weight with a wide molecular weight distribution is used is proposed. As shown in Japanese Patent Laid-Open (kokai) No. H11-154638, a method in which a base polymer is blended with a low-molecular weight compound having a low melting point is proposed.
It has been well known in the past that a novolak resin is cured by intramolecular crosslinking only by heating (SPIE Vol. 469 (1984), p. 72). In it, a crosslinking mechanism that a phenoxy radical, generated in a hydroxy group of a cresol novolak by heating, moves to a connecting methylene group of the novolak resin by resonance, thereby inducing a radical coupling of methylene groups among themselves for crosslinking is reported. In U.S. Pat. No. 3,504,247, a patterning process using an underlayer film having an increased carbon density, which is obtained by a dehydrogenation reaction or a dehydration condensation reaction of a polycyclic aromatic compound such as a polyarylene, a naphthol novolak, and a hydroxy anthracene novolak by heating, is reported.
A glassy carbon film is formed by heating at a temperature of 800° C. or higher (Glass Carbon Bull. Chem. Soc. JPN., 41 (12), 3023-3024 (1968)). However, in view of an effect on device damage or wafer deformation, the highest temperature allowed for heating in a lithography wafer process is 600° C. or lower, and preferably 500° C. or lower.
It is reported that, as a process line width progresses toward narrower, a phenomenon such as wiggling and bending of a resist underlayer film occurs when a substrate to be processed is etched by using a resist underlayer film as a mask (Proc. of Symp. Dry. Process, (2005), p. 11). A phenomenon of displacement of a hydrogen atom in a resist underlayer film with a fluorine atom during substrate etching by a fluorocarbon gas is shown. It is assumed that wiggling of a further fine pattern takes place by volumetric swelling of a resist underlayer film or a lowered glass transition temperature as a surface of a resist underlayer film is changed to a Teflon (registered trade name). In the foregoing Document, it is shown that wiggling can be prevented from occurring by using a resist underlayer film with low hydrogen content. An amorphous carbon film made by a CVD method can reduce a hydrogen amount in the film remarkably well so that it is highly effective for prevention of wiggling from occurring. However, introduction of a CVD method is difficult in a certain case because it is poor in gap filling characteristics of nonplanarity as mentioned before, and the equipment is expensive and requires a large footprint area. If the wiggling problem should be solved by a composition for an underlayer film, which is formable by application, especially by a coating, especially a spin coat method, it would be of great merits because its process and equipment are simplified.
A multilayer process, which involves formation of a hard mask onto a resist underlayer film by a CVD method, is under investigation. In a silicon-type hard mask (a silicon oxide film, a silicon nitride film, and an silicon oxynitride film) too, an inorganic hard mask formed by a CVD method and the like has a higher etching resistance than a hard mask formed by a spin coat method. There is a case when a substrate to be processed is a low dielectric constant film, and poisons a photo resist (i.e., poisoning). In such a case, a CVD film is more effective as a film to block the poisoning.
Accordingly, a process, in which a resist underlayer film is formed by a spin coat method for flattening, and then an inorganic hard mask intermediate layer film as a resist intermediate layer film is formed on it by a CVD method, is under investigation. When an inorganic hard mask intermediate layer is formed by a CVD method, especially in the case of a nitride film formation, it is assumed that heating of a substrate at 300° C. lowest, usually at 400° C. is necessary. Accordingly, when a resist underlayer film is formed by a spin coat method, a heat resistance of 400° C. is necessary. However, not only a usually used cresol novolak or naphthol novolak but also a highly heat resistant fluorene bisphenol cannot endure heating at 400° C., resulting in a substantial film reduction after heating. Accordingly, a resist underlayer film having a heat resistance endurable a heat treatment at a high temperature in the formation of an inorganic hard mask intermediate film by a CVD method is required.
Because of a film reduction and a resin deterioration after heat-treatment due to such a low heat resistance, a heat treatment of a composition for a resist underlayer film has been carried out at 300° C. or lower (preferably in a range of 80 to 300° C.). However, problems such as a film reduction after treatment with a solvent and a pattern wiggling during substrate etching have been still remaining unsolved.
As mentioned above, a method for forming a resist underlayer film having optimum n-value and k-value as an anti-reflection film with good gap filling characteristics, excellent resistances to etching as well as solvent, a heat resistance endurable a heat treatment at a high temperature in the formation of an inorganic hard mask intermediate film by a CVD method and the like, and without wiggling during substrate etching is desired together with a composition for the resist underlayer film usable in such a method.